There are two gestures now essential to understanding South African politics. One is a rolling hand motion as practised by football fans when calling for a player to be substituted. The player they want yanked off is the president, Jacob Zuma.

Both these gestures were prominent, accompanied by much singing and stomping, at Thursday night’s launch of the authorised biography of Kgalema Motlanthe, Zuma’s deputy and his only credible challenger in an African National Congress (ANC) leadership contest in December.

Indeed, this was less a book launch with wine-sipping literati than a raucous anti-Zuma rally attended by top dissidents Tokyo Sexwale, Mathews Phosa and rebels from the ANC youth league. In the absence of US-style presidential debates in South Africa (Zuma v Thabo Mbeki a few years ago would have been an amusing clash of styles), this was the closest thing the rank and file could get to a proper look at Motlanthe.

For it could be argued that Motlanthe’s principal strength in the presidential race is that he isn’t Zuma. The 63-year-old is a private man and opaque politician; few voters would be able identify a clear Motlanthe agenda. He is the tabula rasa on to which the “anyone but Zuma” campaign can project their hopes and, perhaps, wishful thinking.

The publication of his biography, therefore, risks letting in daylight upon magic. At the launch in the great hall at Johannesburg’s Wits University, Ray Hartley, editor of the Sunday Times, said: “I think this book does introduce South Africa to Kgalema Motlanthe for the first time.

“It does give us our first detailed picture of the deputy president. Those of us in the fourth estate have been scratching around to try to work out who is the man who was the third president of South Africa.”

Third president? Yes, Motlanthe was a kind of caretaker president for seven months after Mbeki was ousted and before Zuma won the 2009 general election. But president he was. The record will always show that he wore the crown and so does my memory. On my first day covering Africa for the Guardian, I attended a press conference with President Motlanthe specifically for foreign journalists (more than three years later, we are still waiting for Zuma to do the same).

In fact, South African journalists mining advance copies of the biography are yet to come up with a blockbuster revelation. There have been tales about Motlanthe saving Zuma from expulsion from the party, Motlanthe disagreeing with the decision to expel the youth leader Julius Malema and Motlanthe expressing discontent over various government failings. But still little to fire up voters either way.

“The man is truly a biographer’s delight,” the author, the former trade unionist Ebrahim Harvey, told the audience on Thursday night, implying that Motlanthe will not be losing much sleep over its contents.

Harvey said he had spent almost 200 hours with the politician over three years (adding that Mbeki only gave his biographer 45 hours). “He decided to open up his mind both personally and politically in this biography,” he said. “It was a very courageous step.”

Motlanthe is a man with nothing to hide, he continued. Even when a newspaper falsely claimed that Motlanthe was having an affair with a 24-year-old, not once was he “morose, dejected, looking troubled”, but instead showed “amazing fortitude”.

Harvey insisted that the timing of the book’s publication, just two months before the leadership election in Mangaung, was pure coincidence. But then he took a swipe at the ANC over its response to the deadly unrest sweeping the country’s mines. “The ANC pretence that we don’t have a social crisis in this country is quite ridiculous. In that respect the timing for me is much more important than Mangaung.”

Next up was the man himself – like Zuma, a former inmate on Robben Island. Motlanthe was, as ever, charming and cerebral but not exactly charismatic. “I thought a political biography has got to be written by someone who is going to be critical and make an assessment of the work we do, the positions we adopt and so on,” he said. “I did not want a book that is only about positive issues.”

He then told a story about a trip to Italy where he was impressed by a new teaching method. “Things are always changing,” he observed innocently, prompting the biggest roar of the night from a crowd that could see only the subtext of leadership elections.

Motlanthe paused. Against whoops and whistles, he continued: “For me the irony is already lost because I’m talking about … this is science. It’s not about names of places. I can see that you are trapped within the geographic names of Mangaung (where the leadership election will take place). There are many other important things about Mangaung than just the ANC elective conference.”

He ended on a somewhat sombre note. “I’ve told my comrade Trevor [Manuel, the planning minister], in my will, I leave very clear instructions when I pass, my obituary or tombstone – if anybody believes I deserve a tombstone – should say: ‘Others made suggestions and he implemented.’ “

Which, in the ranks of political epitaphs, is never going to challenge Abraham Lincoln or Winston Churchill. That apart, we still didn’t learn much about the man who would be king (again). The wild cheers seemed to be driven more by out-with-the-old desperation than desire for the new.

“More bloody riddles from the masters of spin, smoke, mirrors and obfuscation,” remarked one journalist on the way out. “What does this guy really stand for except ABZ – anyone but Zuma?”

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